Charles



March 5, 1963 D. CHARLES ELECTRON GUN FOR TRAVELING-WAVE TUBES WITH A TRANSVERSE MAGNETIC FIELD Original Filed March 18, 1950 z M Q LVI/V70 @9N/zz. Cil/Alz- 6.5

United States Patent Office Re. 25,343 Reiseued Mar. 5, 1963 25,343 ELECTRON GUN FR 'liitlAvELiNG-WAVE TUBES WITH A TRANSWRSE MAGNETC EEELD Daniel Charles, Paris, France, assigner to Compagnie Generale de Telegraphie Sans Fil, a corporation of France Original No. 2,694,783, dated Nov. i6, 1954, er. No. 150,357, Mar. 13, 195i). Application for reissue dan.

10, 1961, Ser. No.

Claims priority, application France Mar. 21, 1949 The present invention relates to travelling wave tubes, yand more particularly to a novel electron gun for use in those tubes.

In the United States Patent No. 2,511,407 issued iune 13, 1950, .a travelling-wave tube is described wherein a retardation line comprising two parallel conductors is curved to circular form and `an electric field is applied between its two conductors, while a magnetic field of suitable value and direction is applied at right angles both to said electron beam introduced into the electron and wave interaction space between said two conductors and propagating in `a circle parallel to the retardation line. In the co-pending United States patent application, Serial No. 102,896, filed on .luly 2, 1949, by R. Warnecke et al., a similar arrangement is applied to a system in which the retardation line and the trajectory of the electron beam are not circular but straight. In either oase a cathode for generating the electron beam is placed either at the level of the negative electrode of the retardation line, or in the interaction space between the two conductors, said cathode being raised to a suitable intermediate potential. However, no `arrangement is made to ensure that the electrons, once they have left the cathode and taking into account the presence of the necessary fields for the operation of the system, follow a suitable trajectory as regards position in the system in keeping with the velocity which is imparted to the electrons.

My invention relates to a solution of this problem and provides a source of electrons adapted to generate a beam which is suitable for the correct operation of the tube of the type described.

According to the invention, I provide in the tube, before the input end of the retardation line, an elect-ron gun comprising an auxiliary focusing system of two electrodes between which an electric field is set up which is parallel to the field between the conductors or the retardation line and the value of which is equal to half that of the latter field. I position the emitting cathode of the gun in relation to this auxiliary system at such a level that the electrons penetrate into the interaction space between the conductors of the retardation line at a desired velocity, `approximately equal to the phase velocity of the electromagnetic wave which is excited in the conductors of the retardation line and which is to undergo an amplicaticn in the tube by interacting with the electron beam. Furthermore, I make the position in space of the auxiliary system with respect to the conductors of the retardation line such that the imaginary equipotential surface along which the electrons leave the auxiliary system merges into the imaginary equipotential surface that corresponds to the desired velocity of the electrons in the field between the conductors of the retardation line.

The invention will be more clearly understood by considering the non-limitative embodiment thereof illustrated in the single figure of the accompanying drawing.

This figure shows an axial section, in the plane at right angles to the lines of magnetic field, of a travellingwave tube described in the aforesaid patent and application, and more particularly of a tube of straight shape. lt is, however, to be understood that everything stated in the ensuing description is also applicable to tubes of cylindrical shape, in which the distance between the conductors of the retardation line is small .as compared with the radius of the cylinder, so that it may be considered that everything takes place as if the structure were flat.

The tube, which is contained in an envelope 1 of insulating material, comprises the usual elements of a travellingwave tube with a transverse magnetic field, viz. aline comprising two conductors 2 and 3, the conductor 2 for example of which is a retardation line shown schematically in the form of a helix raised to a positive potential, while the conductor 3 is a fiat negative electrode. A collector 4 is provided for collecting the electron beam, and the coaxial lines 5 and 6 respectively serve for exciting the line yand for collecting the amplified energy. Magnetic poles not shown, located on either side of the tube, supply a magnetic field at right angles to the plane of the drawing, a few of the lines of force of which are shown at 7. lf the electric field between the conductors 2 and 3 is directed upwards in the figure, and if the electrons are to move from left to right, the magnetic field must be directed from the observer towards the drawing. The electric eld between the conductors of the line is set up by means of the source S that supplies the collector 4 and the retardation line 2 by means of its positive pole which is connected to ground.

By the combined action of the electric field and the transverse magnetic field, the electrons, under optimum conditions, describe in the space between the conductors 2 and 3 a trajectory at a mean velocity of' the wave in the line. In order that said trajectory, in the absence of the radio-frequency field, may be a cycloid described along a line parallel to the retardation line, it is necessary for the electric field E and the magnetic field B to -be related to each other by the equation To this velocity there corresponds in the field an equipotential level or imaginary surface 9, the potential Vl, of which is determined by the kinetic energy equation:

6Vn= rnv.2 (2) The conditions for stability require that the electrons be introduced into the field E at the velocity v precisely along the equipotential surface 9, in order that they may yfollow a mean straight path at the level of that surface.

On the other hand, the potential to be given to the retardation line is determined by the considerations of power efiiciency which is substantially equal to the ratio between the difference of potentials through which the electrons pass laterally by the action of the radio-frequency field (difference of potentials between the conductor 2 and the equipotential surface 9'), and the difference of potentials between the conductor 2 and the equipotential surface zero (the potential of the source of electrons being taken by convention as 0), i.e. the difference of potentials between the conductors 2 and 3 if the conductor 3 is at the potential ofthe cathode. ln other words, if the potential of the conductor 2 is denoted Iby nVo, the efiiciency will be Itallas-,tantially equal to If a desired efficiency is given, I therefore have all the elements for determining the potential of the conductor 2, lthe electric field and the magnetic field to 4be applied to .l was defined by thefEquations @that an' efficiency of the system, and alsofor ldefining the position of the equipo'tential surface 9 in said system.

According to the invention, I provide before the entrance into the syste-m. of conductors 2 and 3, an auxiliary system of two beam focusing electrodes andll which are formed for example byplates parallel to theaxis of ,the systemand to the magneticl field, and' between which electric field v is set up (the fieldv E being the field between the electrodes vZand A3). The emitting cathode12, which is heated by a fila-ment 13 supplied from a source14, is located for example'at thelevel'of the' electr-odell which is this case is connected to the potential of thev cathode'12assumed by definition to be 0. In order to obtained the desired field, rit is' onlynecessary to connect the electrode 10 byv means 'ofthe connection 1S toan intermediate' potential obtained from the source 8 thaty supplies' the conducto-r 2. The conductor Sis connected in the exa-mple described to the elec- Strode 11, i;e..it is at' the potential 0,' without this 4arrangel ment being limitative`,-since `said electrode mayalso be fraised to a certain negative or positive potential provided fthat it: isishifted transversely `a suitable Idistance relatively to the' equipotential surface 9.

In the Aarrangement shown,` anielectron that starts with- "1 out initial velocity from the'level ofthe' cathode at the cpotential 0, describes, in the presence of a-perpen'dicular lmagnetic-field, y.a cyclo'id-l `at `a-velocity equal to:

wherein El is the electric; fieldv "and 11.

t It-jcan be seenthatfif between they plates `F10 n Owingto the fact thatthis velocity, which' may bereph resented by a; line A'tangent to thecycloi ,isy parallel tothe axis of the system at the highest point ofthe trajectory 16, it can be seen that if said highest point is located directly 'at the outlet from the system 10-11 and at the inlet the system 2 3, I have obtained the object that my inven- .tion kis intended to provide,'viz." that the electrons penef trate into ythe space between the vconductors of' the 'retardation lnei'at a velocityiequal wave to be amplified. v

i Furthermore, in order 'to obtain a smooth join Without sudden variations of field, I shift the Isystem v10a-f1 Atranstothe phase ivelocity of the verselyl with respect'tdthefsystem 2*-3 fin such a manner `f that the 'equipotential surface tronsleave the system 104-11,

17, along A'whichthel elecis joined to the equipotenwhichV inf thersystem 2 3 l and 2 hereinbefore exfplained. Since the equipotential surface 9' corresponds rtialfsurface 9, the position ofy `to the potential V0,`the equipotential surface `17 'correrponds to the axis of symmetry between 1() land'll, the Apotential to bek given tothe electrode 10 is 2V, and the 'lfdistance d1 between 10 and 11 is defined by thecondition "Since, on the other hand, the distance d between 2 and: 3

is given by "clu'ding a delay line, 4 1 line and defining therewith 'an interaction'pspace, meansffor 25` jmeans for providing /t scid interaction'spq'ee afmagneytic dield, said electric and magnetic fields beingk perpendicular `electrode and vdening a l vidingin said focusing spacca` uniform electric field js'nb- "jjstantially less than-the eleetrie'field'fin isaid interaction 40 fspace, of same ldirection 'said electrodes for Vto the expert using the general principle which comprises placing the emitting cathode of a tube of the type described in an auxiliary focusing system in which there 1s `any electric field which is substantially less than the field required for the operation of the tube, said system being arranged relatively to the overall structure of the tubein such a manner that there is a gradual joining of theV two fields. As shown in the drawing, this latter condition is obtained when the distance R betweenthe equipotential surface in the interactionspace and a parallel plane P passing through the cathode is equal to the algebraic sum of the Vdistance d2 between the conductor l3 and the equipotential surface of same potential value in the focusing space and the distance d3 between the conductor 3 and the plane `P.

l. A crossed-field travelling wave tube ofthetype vina base electrode panallelto said y`delay providing in said interaction spacean"electricf'eldfand to eachother toform crossed jieldsjand tothe directionl'of said delay' line and' base electrode; wherebyk therefrisjprovided `within said interaction space 'an equipotential "[plane] surface correspondingto` the rneanvelocity lwith which electrons,y when fed into said space, propagate therein` under theaction of' said crossed Ields, saidtube' 'comprising: means' comprisingy a' cathode and two v'electron focusing electrodes parallel to fsaid delaylinetand base focusing space, ;means *topproand' parallel thereto; and'` fortpr'o- Vviding'theV same lmagnetic field therein' asimsaiduinter- 'action' space, said cathode being ,positioned to feed `electrons into said focusing space in al direction `plerpenditnil'ar to said focusing electrodes and parallel to' the'jdirection' of said second-mentionel` electric field, the"distance between the center of said cathode and the'plane of' entry'of said electrons into said interaction space being' substantially equal to half the chordof an arch ofthecycloidal ,path yof said electrons in said focusing space under` the action of said fields, the distance between said equipotentiai'lfplan] surface and a parallel [plane] surface passingthrough said 'cathode' being equal to the algebraicjsum ofjthefdistance betweenv said base electrode :and the'equipotenti'al )plane ofsarne potential' valueinjtheffocusing Vspace and of the distance between 'said base electrode Vand saidfparalle] plane, and rvsaid focusing electrodes beingrposi'tione'd for delivering said electrons Ainto said interaction :space `as soon as they leave said focusingspae. I y Y V v 2. In a travelling wave tube having a retardation" line and a parallel spaced' conductor defining yan 'interaction space therebetween, terminal connections thereto'for applying a potential difference therebetween lo provide on electrostatic field in said interaction space,V and means'for establishing a magnetic field in the interaction space having its lines of force directed at right angles'to [the] said electrostatic field and to the direction ofthe line, whereby there is provided within said interaction space an: equipotential [plane] surface corresponding to the `mean velocity with which electrons, whenv fed into saidl space, propagate therein under the action of said Vcrossedfields, an electron gun comprising a pair of parallelspaced Jelectrodes defining a` focusingV space therebetweenl communieating with the interaction space; terminal connections to applying a' potential difference therebetween, an electron emissive cathode positioned toemit electrons into said focusing space and `contained'n said magnetic eld, whereby the electrons emitted by said cathode are caused to follow a cycloidal trajectory starting from said cathode, the magnitude of the potential difference and the distance between said electrodes being such that the potential in -said focusing space at the level of the highest point of said cycloidal path is equal to the potential in the interaction space at said level, said highest point being contained in a transverse plane common to said spaces and defining for the electrons an exit from said focusing space and an entry into the interaction space, and the distance between said level and a parallel plane passing through said cathode being substantially equal to the algebraic sum of the distance between said conductor and the corresponding equipotential [level] surface in said ocusing space and of the distance between said conductor and said parallel plane.

3. In a traveling-wave tube, the combination according to claim 2, wherein the potential dierence between said pair of parallel spaced electrodes dening the focusing space is one half the electrostatic field in said interaction space.

4. In a traveling-wave tube, the combination according to claim 2, wherein said electron emissive cathode is at the same potential as one of said pair of parallel spaced electrodes.

5. In a traveling-wave tube, the combination according to claim 2, wherein the distance between said pair of pazrallel spaced electrodes is substantially equal to the distance between said retardation line and said conductor.

References Cited in the le of this patent or the origlnal patent UNITED STATES PATENTS 2,232,158 Banks Feb. 18, 1941 2,372,328 Labin Mar. 27, 1945 2,414,121 Pierce Jan. 14, 1947 2,511,407 Kleen et al. June 13, 1950 2,531,972 Doehler et al Nov. 28, 1950 2,600,509 Lerbs June 17, 1952 20 2,607,904 Lerbs Aug. 19, 1952 

